From ad78b81a1077f7d956952cd8bdfe1e61504e3eb8 Mon Sep 17 00:00:00 2001
From: Cristian Marussi <cristian.marussi@arm.com>
Date: Thu, 22 Dec 2022 18:38:20 +0000
Subject: [PATCH] firmware: arm_scmi: Harden shared memory access in fetch_response
Git-commit: ad78b81a1077f7d956952cd8bdfe1e61504e3eb8
Patch-mainline: v6.2-rc5
References: git-fixes

A misbheaving SCMI platform firmware could reply with out-of-spec messages,
shorter than the mimimum size comprising a header and a status field.

Harden shmem_fetch_response to properly truncate such a bad messages.

Fixes: 5c8a47a5a91d ("firmware: arm_scmi: Make scmi core independent of the transport type")
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20221222183823.518856-3-cristian.marussi@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 drivers/firmware/arm_scmi/shmem.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/firmware/arm_scmi/shmem.c b/drivers/firmware/arm_scmi/shmem.c
index 1dfe534b8518..135f8718000f 100644
--- a/drivers/firmware/arm_scmi/shmem.c
+++ b/drivers/firmware/arm_scmi/shmem.c
@@ -81,10 +81,11 @@ u32 shmem_read_header(struct scmi_shared_mem __iomem *shmem)
 void shmem_fetch_response(struct scmi_shared_mem __iomem *shmem,
 			  struct scmi_xfer *xfer)
 {
+	size_t len = ioread32(&shmem->length);
+
 	xfer->hdr.status = ioread32(shmem->msg_payload);
 	/* Skip the length of header and status in shmem area i.e 8 bytes */
-	xfer->rx.len = min_t(size_t, xfer->rx.len,
-			     ioread32(&shmem->length) - 8);
+	xfer->rx.len = min_t(size_t, xfer->rx.len, len > 8 ? len - 8 : 0);
 
 	/* Take a copy to the rx buffer.. */
 	memcpy_fromio(xfer->rx.buf, shmem->msg_payload + 4, xfer->rx.len);
-- 
2.35.3